The broad definition of memory consolidation is the process by which recent memories are crystallised into long-term memory. The word "consolidation" is used to refer to different levels:

Molecular consolidation: The molecular process by which long-term conductivity of synapses is affected. Memory consolidation occurs after training (e.g. an exposition to a stimulus-response pair). Consolidation increases in strength over time with repetition. Maximum consolidation with minimum time investment is achieved by means of spaced repetition. Molecular consolidation requires protein synthesis.

Network consolidation: Many researchers believe that episodic memories are initially stored in the hippocampus and are slowly moved (or 'consolidated') into the neocortex. This process of consolidation begins during wakefulness and may be enhanced during sleep. Originally it was thought this happens during dreaming (Marr, 1971). However, new research indicates that the NREM phase of sleep is associated with that process (Hobbson, Stickgold, Buzsaki).

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Reconsolidation

There is evidence in laboratory animals that recall puts memories into an unstable, labile state and that, after recall, the memory must be re-consolidated or it will be forgotten. Both consolidation and reconsolidation can be disrupted by pharmacological agents (e.g. the protein synthesis inhibitor anisomycin) and both require the transcription factorCREB. Recent research suggests that BDNF is required for consolidation (but not reconsolidation) whereas the transcription factor and immediate early gene Zif268 is required for reconsolidation but not consolidation. Memory re-consolidation occurs upon review or repetition of the learned material.